1. Field of the Invention
This invention relates to an electric motor running system employing a photovoltaic array wherein output voltage from the photovoltaic array is invereted to AC voltage by a frequency-controllable inverter to thereby apply the AC voltage to an induction motor, and more particularly to such a system which includes a means for providing the induction motor with starting compensation in the initial starting and/or restarting after stop during running.
2. Description of the Prior Art
In the photovoltaic array, the voltage drops to a large extent with increase of an amount of current as compared with an alkali and a lead storage batteries. In FIG. 14, the solid line shows the output voltage (V)-output current (I) characteristics of the photovoltaic array in the case where the solar insolation degree Ee at 28.degree. C. is a parameter. The broken line denotes the output voltage (V) -output power (W) characteristics in the above-mentioned state of condition. FIG. 14 shows that the output voltage is gradually decreased to zero with increase of the amount of the output current in the photovoltaic array. On the other hand, the output voltage is increased with decrease of the amount of the output current and at last reaches an open-circuit voltage (or no-load output voltage). This shows that the photovoltaic array has wide voltage fluctuations. Accordingly, where the photovoltaic array is employed as a power source for an electric motor for driving a pump, fan or the like, a large amount of current flowing at the starting of the motor causes excessive voltage drop, which renders the starting of the motor impossible. In order to prevent this, the photovoltaic array having a larger capacity than that needed for normal running of the motor is employed. This, however, causes the photovoltaic array to become large in size, which brings about rise of the production cost.
To prevent the above-described case, the prior art has provided a device which has an inverter between the photovoltaic array and the motor. The inverter has functions to invert DC voltage to AC voltage and to control the output frequency. According to this device, the output frequency of the inverter is automatically controlled during the starting of the motor so as to be gradually increased to the normal running frequency, preventing excessive increase of the starting current so that the motor is started smoothly.
Furthermore, the inverter is controlled so that the torque of the motor may become constant in a predetermined range of rotation, maintaining a constant ratio of the output voltage to the output frequency thereof. In addition, the inverter is automatically controlled so that the output voltage of the photovoltaic array may be maintained at a predetermined value Vc, in order to prevent the output voltage of the photovoltaic array from being varied due to the load variations. In this case, the value Vc is determined so that the output power of the photovoltaic array may become maximum at the value Vc under different solar insolation degrees, as shown in FIG. 14, whereby the photovoltaic array is normally used at the maximum efficiency.
According to the above-described device including the inverter, where a load connected to the electric motor has a large static friction torque as in the case of a positive displacement pump instead of a centrifugal pump or a fan, the load starting torque becomes excessively larger than the normal running torque of the motor which is attained at a normal running frequency for maintaining the voltage Vc. Consequently, in the case where the inverter is controlled during the starting of the motor so that the output frequency of the inverter is gradually increased to the value of the normal running frequency, the motor is led into the condition where it is unable to be started.
The above-described problem also arises in the case where the motor is automatically restarted after temporary stop during running due to drop of the solar insolation degree.